In the past, most small-scale; i.e. less than 250,000 SCFD, hydrogen users relied on a liquid hydrogen storage supply or delivery of high pressure gaseous hydrogen via tube trailers to supply hydrogen at the desired site. On-site liquid hydrogen storage tanks have proved to be very inefficient, many resulting in a boil-off of about 0.2% per day. This results in an inflated unit cost of hydrogen, especially when the hydrogen is stored on-site over a relatively long period of time. Delivery of the liquid hydrogen supply by tube trailer also results in an increased hydrogen cost due to liquefaction, travel and delivery costs, especially if the desired point of use is located at a considerable distance from the hydrogen production plant.
Small-scale users have employed ammonia dissociation methods to produce hydrogen from an on-site liquid ammonia storage tank. While this method overcomes the problem of liquid hydrogen boil-off, the purity of the hydrogen produced is generally too low and must subsequently be further treated in a PSA (pressure swing adsorption) unit. The use of an ammonia dissociation unit combined with a PSA unit typically results in about a 25% loss of the total hydrogen. An alternative method was also developed using an on-site supply of methanol which could be dissociated as the hydrogen is needed. As with the ammonia dissociation method, the hydrogen must undergo further purification steps, such as treatment in a PSA unit, which results in a large product loss.
An article by G. Strickland entitled "Hydrogen Derived from Ammonia: Small-Scale Costs", Int. J. Hydrogen Energy, Vol. 9, No. 9, pp 759-766 (1984) states that hydrogen derived from anhydrous liquid NH.sub.3, via a dissociator and H.sub.2 purifier, offers an alternative to conventional methods of obtaining pure H.sub.2 for small-scale use. The specific process outlined in the article employees a polybed pressure swing adsorption (PSA) system for the H.sub.2 purification step. It is stated that when using this purification step in conjunction with ammonia dissociation, about 75% of the hydrogen could be recovered with a fuel credit obtained for the remainder.
European patent application No. 83306428.0 discloses a method and apparatus for the production and delivery of hydrogen, especially adapted for on-site production for hydrogen users requiring on the order of 28 to 2800 m.sup.3 of hydrogen per day. According to the disclosure, hydrogen is produced by first dissociating ammonia in a typical dissociation reactor, and subsequently passing the ammonia feed stream to a bed of hydridable material which exothermically and selectively adsorbs hydrogen from the feed stream and endothermically desorbs hydrogen on demand. The H.sub.2 purification hydride system comprises at least one flow through reactor having inner and outer heat exchange shells and a bed of hydridable material located co-axially there between. Additionally, a means for circulating fluid through the heat exchanger shells whereby heat may be extracted therefrom during adsorption of hydrogen from the feed and whereby heat may be supplied thereto when hydrogen is desorbed from the hydride bed. It is stated that the heat transfer characteristics of the flow-through reactor of the disclosed invention are at the heart of its performance. The heat flow is interrelated to flow rate, pressure and recovery and those variables operate to establish the effective hydrogen adsorption pressure.
Sheridan, et al. U.S. Pat. No. 4,360,505 discloses an improved adiabatic process for separating hydrogen from mixed gas streams using hydridable materials as the adsorbing medium. The improvement involves utilizing a composite comprising a thermal ballast in admixture with the hydride material to adsorb the heat of the reaction and to aid in desorption. By virtue of the intimate contact of the ballast with the hydridable material, rapid cycle times plus good bed utilization are achieved.